The invention relates to the field of triboelectricity, specfically a triboelectric device with variable and adjustable polarization.
Triboelectic devices can be used, for example, in the electromechanical transducers used in sonar antennas. These transducers may be hydrophones; accelerometers; detectors of hydrostatic pressure, deformation, force or temperature; or complex systems combining these different functions. These devices may also be used in the air in microphones, accelerometers, telex or computer keyboards, heat detectors, or coatings that change colors under applied pressure.
Most transducers used in industry are piezoelectric material and particularly composite piezoelectric material.
U.S. Pat. No. 2,420,864 describes a process for producing a piezoelectric material comprising a plastic matrix and a single plezoelectic crystal.
The plastics quoted are celluloids, cellulose acetates, chlorinated rubbers, phenol-formaldehydes, phenol-furfurol resins, acrylates, methacrylates and polystrenes, while the single crystals are chosen amongst Rochelle salt, tourmaline, saccharose and tartaric acid.
These materials are intended for use only in hydrostatic mode, in other words with their entire surface exposed to the incident sound pressure. However, these single crystals have very low hydrostatic constants and are therefore not very effective.
Single crystals have progressively been replaced by perovskite ceramics and more particularly by lead zircon titanate (PZT) or barium titanate, so that high emission powers can be used. For reception purposes however, their hydrostatic constant is less than that of single crystals, such as lithium sulphate, and their use is therefore limited to particular compression modes belonging to types 33, 31 or 32.
U.S. Pat. No. 4,233,485 describes a composite piezoelectric material consisting of ceramic powder and polymer. This powder is a sintered mixture of lead titanate and double iron/bismuth oxide with a mean grain size of 5 microns, while the weight contribution of the double iron/bismuth oxide ranges from 50 to 80%.
U.S. Pat. No. 4,407,054 describes a composite piezolectric material consisting of a prepolymer, such as a mixture of castor oil and ricinoleate isocyanate, and a piezoelectic material such as quartz, PZT, tourmaline, tartaric acid or lithium sulphate
Finally, U.S. Pat. No. 4,868,856 describes a composite piezoelectric material consisting of a polymer produced from polyurethane and a piezoelectric material such as tartaric acid, a monosaccharide, a disaccharide, or a carboxylate.
It is also known that the products known as xe2x80x9cElectretsxe2x80x9d, obtained by subjecting waxes liquefied by heating to a strong electrical field during their cooling phase, exhibit pronounced and permanent pyroelectric and piezoelectrc properties.
All these materials have a number of disadvantages, notably when they are used in hydrophones. In fact their polarization and, more generally, their piezoelectic properties, are fixed during manufacture and they consequently have a single or specialized mode of operation (hydrostatic mode, mode 33, 31, 15, and so on). For example a composite produced by mixing grains of tartaric acid or lithium sulphate with an unpolymerized resin, the grains then being polarized using one of the processes described in the earlier patents, can be used only in hydrostatic mode, and its piezoelectric properties are, in principle, invariable or vary slowly with time.
One of the purposes of the present invention is to obtain a material whose properties can be varied as required by applying an electric field when the transducer including this material, or the antenna comprising the transducers using this composite material, is in service. Thus, for example, when an antenna becomes faulty, it is possible to use a material according to the invention in the antenna to correct the fault. It is therefore unnecessary to take action involving the receiver, which is not always possible, or which increases the cost of the receiver.
The antennas may also be exposed to interfering noise or to continuous or transient spurious signals. The invention makes it possible to eliminate or reduce these interfering noises and spurious signals by directly modifying the characteristics of the elementary sensors.
For this purpose, the french patent FR9404853 describes a piezoelectric device including means for applying a polarizing voltage to a material, said means comprising a voltage source, a polarizing amplifier and electrodes fixes to the material, and preferably, means of adjusting the value of the polarizing voltage.
Many steps are necessary to manufacture such a device.
It is a main object of the invention to provide a transducer having the same functionality as the one described in the french patent FR9804853 but with an increased sensitivity and far simpler to manufacture.
The Present invention relates to a triboelectric device comprising means for applying a polarizing voltage to a first material, said means comprising a voltage source, a polarizing amplifier and electrodes wherein the first material is a polymer or dielectric organic substance, which is piezoelectic not piezoelectric or slightly piezoelectric, and the electrodes are not fixed to the first material, allowing relative rubbing displacements between the electrodes and the first material.
According to a particular embodiment a conductive material, called a second material in the following, is interposed between the electrodes and the first material.
According to an additional embodiment, the second material is petrolatum.
According to a particular embodiment the polymer or the dielectric organic substance is chosen from the polystyrenes, polysulfones, polycarbonates, or polypropylene, polyethylene, the polyethylene terephthalates, polyphenylene oxides, polyacrylate esters, polymethacrylate esters, polyvinyl chlorides, polyvinyllidenes, acrylonitrile polymers, methacrylonitrile polymers, polyvinyl acetates, celluloids, cellulose acetates, polyvinyl pyrrolidones, cellulosic polymers, phenol formaldehydes, phenol-furfural resins, soluble polymers of polyimide, epoxy resins, polymerisable resins, natural rubbers, synthetic rubbers, silicone resins, methanol or bone glues, polystyrols, gum-lacs, copolymers of vinylidene and trifluoroethylene.
According to a particular embodiments, said means for applying a polarizing voltage to a first material comprises means of adjusting the value of the polarizing voltage.
Furthermore the polarizing voltage may be continous or alternating.
According to a particular embodiment, these electrodes are of the comb type.
According to an another embodiment, the first material comprises particles such as fibers, powders or fragments of objects or materials which are, according to circumstances, piezoelectric, slightly piezoelectric or even non-piezoelectric.
According to another embodiments, the size of the particles is between 5xc3x9710xe2x88x929 m and 5xc3x9710xe2x88x923 m.
According to a particular embodiments, particles are wholly or partly chosen from a biological material or from sheets or wafers of piezoelectric plastics materials that are either prepolarized in the through-thickness direction or not prepolarized, or from microcrystals.
According to a preferential embodiment, the biological material is chosen the oligosaccharides, the homopolysaccharides, the heteropolysaccharides, the glycosaminoglycans, the proteoglycans, terpenes, steroids, amino acids, oligopeptides and polypeptides, proteins, nucleotides or nucleic acids, urea or natural silk.
According to another embodiment, particles are, wholly or partly, microcrystals or fibers of ammonium tartrate. potassium tartrate, ethylene diamine tartrate, dipotassium tartrate, sodium chlorate, sodium bromate, nickel sulphate hexahydrate, iodic acid, benzophenone, hydrated or dehydrated lithium gallate, polyvinyl difluoridine, polyvinyl chloride, copolymer of vinyl acetate and vinylidene cyanide, copolymer of vinylidene and trifluoroethylene, polyvinyl fluoride, nylon, polyacrylonitrile, polyparaxylene, polybichloromethyloxetane, aromatic polyamide, polysulfone, cyanoethylcellulose, monosaccharides, disaccharides, brushite, monetite or salts of carboxylic acid.
In this way it is possible to obtain a bimodal or polymodal material, i.e., a material which, depending on the nature of its piezoelectric components, may be for example used either in mode 33, or in hydrostatic mode, the mode of operation depending upon the field applied to it.
By using at least two devices according to the invention, one can obtain a sensor operating in two different modes.